PPA Headphone Amplifier

The PPA headphone amplifier was designed by Team PPA in
late 2002 as a community service project. Over the past few years, PPL
created a series of pocket
headphone amplifiers using opamps and open loop buffers which
were
the inspiration for Apheared's
#42 and the META42.
The PPA incorporates a mixture of technologies from several PPL pocket
amps, the META42, and new techniques such as differential output. PPA
stands for PPL's Portable Amplifier, in honor of PPL's numerous
contributions to the headphone amplifer community. PPA v1.1 was
designed in early 2004. PPA
v2
replaced v1.1 in March 2005. Team PPA has disbanded.

Differential Output

The PPA has 3 amplifier channels (left, right, and ground) which use
the same output buffers and noninverting opamp topology. The ground
channel sources and sinks the return current from both drivers which
would otherwise have been dumped into signal ground or power supply
ground. This shifts responsibility for the high current reactive load
of
the headphones from signal ground to the supply rails of the ground
channel buffer, thus removing the primary source of signal ground
contamination. The drivers have symmetrical output buffers with equal
impedance and transfer characteristics on both sides, rather than an
output buffer on one side and the large capacitor bank of the power
supply ground on the other. This results in lower output impedance and
greater linearity.

Standard headphones have 3 wires: left, right, and ground, which is
tied to the negative side of both drivers. Standard headphone
amplifiers
have 2 channels: left and right, with signal ground or power supply
ground used for the return path from the headphone ground wire. Fully
balanced headphone amplifiers have 4 channels, require custom 4 wire
headphones, and are typically operated in bridged mode, which doubles
the output voltage and quadruples the power by using inverting and
noninverting pairs, but does not offer symmetrical transfer
characteristics due to the difference between inverting and
noninverting
modes.

Jung Multiloop

Jung multiloop reduces distortion and increases stability with a
high gain inner or local loop and sets the overall amplifier gain with
an outer or global loop. There is no significant load on the opamps so
they operate with low distortion and confer their linearity to the
unity
gain open loop output buffers which drive loads with greater stability
and lower distortion than closed loop buffers or power opamps.

FET Isolated Power Rails

The opamps are powered by their own FET isolated rails so audio
frequency modulation on the output buffer power rails does not get into
the signal. A TLE2426 splits the opamp rails and references signal
ground but not output ground.

FET Cascode Current Source

The opamps are biased into Class A operation using a FET cascode
current source which improves sound quality by reducing transient
intermodulation distortion and operating further into the linear range
than Class AB or B modes. The source resistor RS
(R9)
trimpot allows for more precise current control, compensation for wide
FET IDSS
range, and the use of higher IDSS FETs to source
lower
currents.

Bass Boost Calculator

R3 = kΩ,
R4 = kΩ,
R7 = kΩ,
C7 = µF

JavaScript
is required.

LED Power and Battery Life Indicator

There are several possible LED configurations. RLED
is a
cheap and simple option for AC only amps. A FET current source gives a
more professional appearance when powering down as the LED will not
fade
gradually. A current source and Zener maintain constant LED brightness
until battery voltage drops to a preset level determined by the Zener,
at which point the LED goes out, indicating it is time to recharge the
batteries. A FET with RFET (source resistor)
allows any FET
to
be used, but RFET must be hand selected to set
the current.
If
RFET is jumpered the FET must be hand selected
for low IDSS.
If constant LED brightness is desired without low battery indication,
the Zener may be jumpered. A CRD is pricey but convenient as the
current
rating is precise. A pad is provided to use a CRD in the RLED
position.

BC Components
was purchased by Vishay,
who broke all web links to BC parts. Hopefully this will be fixed
soon.

See Tangent's
PPA Pages
for detailed parts and assembly information. Note that Tangent's
opinions on "optional" parts are not necessarily the opinions of the
rest of Team PPA. Omitting parts may result in inferior performance and
instability.

Wiring Considerations

Signal ground is input ground, not output ground. Output ground is the
output of the ground channel. It exists solely to drive headphones. It
is not a true ground. Do not connect signal ground to output ground.
This defeats the purpose of differential output and may cause amplifier
instability. Do not use the headphone output as a line level output as
this may short signal ground to output ground. Use signal ground
instead
of output ground for line level outputs.

Many sources tie signal ground to AC ground, either directly, or via
another component in the system. The amp ties signal ground to a vitual
ground created halfway between the power supply rails, so an isolated
power supply must be used. Do not use a power supply with an AC
grounded
output, as this will short one of the power rails to signal ground if
the amp is connected to an AC grounded source, possibly damaging the
amp.

Connect the case and pot housing to signal ground. This helps shield
the amp from hum and noise. Most cases are tied to signal ground and/or
AC ground. Do not connect V+ or V- to the case instead of signal ground
as it will short the rail if it touches another case, cable plug, or
other grounded conductor. Do not connect AC ground to signal ground or
the case. This may cause ground loop problems. For more information: